There are two major conditions that affect an individual's ability to focus on near and intermediate distance objects: presbyopia and pseudophakia. Presbyopia is the loss of accommodation of the crystalline lens of the human eye that often accompanies aging. In a presbyopic individual, this loss of accommodation first results in an inability to focus on near distance objects and later results in an inability to focus on intermediate distance objects. It is estimated that there are approximately 90 million to 100 million presbyopes in the United States. Worldwide, it is estimated that there are approximately 1.6 billion presbyopes.
The standard tools for correcting presbyopia are reading glasses, multifocal ophthalmic lenses, and contact lenses fit to provide monovision. Reading glasses have a single optical power for correcting near distance focusing problems. A multifocal lens is a lens that has more than one focal length (i.e., optical power) for correcting focusing problems across a range of distances. Multifocal optics are used in eyeglasses, contact lenses, and intra-ocular lenses (IOLs). Multifocal ophthalmic lenses work by means of a division of the lens's area into regions of different optical powers. Multifocal lenses may be comprised of continuous surfaces that create continuous optical power as in a Progressive Addition Lens (PAL). Alternatively, multifocal lenses may be comprised of discontinuous surfaces that create discontinuous optical power as in bifocals or trifocals. Contact lenses fit to provide monovision are two contact lenses having different optical powers. One contact lens is for correcting mostly far distance focusing problems and the other contact lens is for correcting mostly near distance focusing problems.
Pseudophakia is the replacement of the crystalline lens of the eye with an IOL, usually following surgical removal of the crystalline lens during cataract surgery. For all practical purposes, an individual will get cataracts if he or she lives long enough. Furthermore, most individuals with cataracts will have a cataract operation at some point in their lives. It is estimated that approximately 1.2 million cataract surgeries are performed annually in the United States. In a pseudophakic individual, the absence of the crystalline lens causes a complete loss of accommodation that results in an inability to focus on either near or intermediate distance objects.
Conventional IOLs are monofocal, spherical lenses that provide focused retinal images for far objects (e.g., objects over two meters away). Generally, the focal length (or optical power) of a spherical IOL is chosen based on viewing a far object that subtends a small angle (e.g., about seven degrees) at the fovea. Unfortunately, because monofocal IOLs have a fixed focal length, they are not capable of mimicking or replacing the eye's natural accommodation response. Fortunately, ophthalmic devices with electro-active elements, such as liquid crystal cells, can be used to provide variable optical power as a substitute for the accommodation of an damaged or removed crystalline lens. For example, electro-active elements can be used as shutters that provide dynamically variable optical power as disclosed in U.S. Pat. No. 7,926,940 to Blum et al., which is incorporated herein by reference in its entirety.